When to Eat

October 15th, 2021 | Posted in Info

When to Eat

If you’ve read the blog post How Much vs What We Eat and Weight Gain you’ll have an insight into the fact that how much we weight is more complicated than might previously have been thought.

The balance between weight gain and weight loss may be predominantly determined by what you eat, how much you eat, and how physically active you are, but there may be yet more things to take into account.

New research shows that it’s not just how many calories you eat, but WHEN you eat them that will determine how quickly those calories are used up.

The Circadian Rhythm

Our daily biological clocks and sleep regulate how the food we eat is metabolised; thus the choice of burning fats or carbohydrates changes depending on the time of day or night. Naturally our circadian rhythm programmes the body to burn fat when we sleep.

Following a meal most of the energy our bodies need comes from glucose from food. After any period of not eating, such as during the night, mobilisation of fat occurs, so that by the morning most of the energy needed by the body comes from fatty acids rather than glucose. We literally burn fat while we sleep. This means that eating late in the evening could delay this fat burning.

To test this researchers monitored the metabolism of middle-aged and older subjects in a whole room respiratory chamber over two separate 56-hour sessions, using a random crossover experimental design. In each session, lunch and dinner were given at 12:30 and 17:45 respectively, but the timing of the third meal differed between the two halves of the study. In one of the 56-hour bouts, the additional daily meal was presented at 8:00am whereas in the other session, a nutritionally equivalent meal was given at 10pm. The duration of the overnight fast was the same for both sessions (1).

The same amount and type of food were eaten in both sessions and the participants activity levels remained the same. But the timing of nutrient availability flipped a switch in their fat/carbohydrate burning such that the late-evening snack resulted in less fat being burned overnight compared to the breakfast session. The timing of our meals therefore affects the extent to which what we eat is used or stored as fat.

This suggests that not eating late in the evening is important for allowing the body to burn fat during the overnight fast.

See also Weight and the Gut Microbiome to find out how the organisms that live inside us affect our ability to lose weight.


1. K Parsons Kelly et al Eating breakfast and avoiding late-evening snacking sustains lipid oxidation. PLOS Biology, 2020; 18 (2): e3000622